Coherent acousto-optic channelizers are employed in signal processing applications, where the signal of interest is typically a pulsed or continuous wave radar signal having a frequency on the order of several or more GHz. For this purpose, as shown diagrammatically in FIG. 1, the general signal processing architecture of such a channelizer includes an input port 11 to which the input signal S(t) of interest is coupled.
As a non-limiting example, the input port 11 may be coupled to the analog signal output of a passive radar receiver (including radar antenna and associated low noise amplifier (LNA) units, not shown), which receives RF signals falling in a 1-5 GHz range.
The RF signal S(t), containing P unknown frequency components, at input port 11 is applied to a transducer 21 of a `signal` acousto-optic modulator or Bragg cell 20, disposed in a `signal` leg or path portion 31 of a collimated coherent light beam derived from beam (spot)-shaping optics 29 and a coherent optical beam generator (laser) 30. The beam spot-shaping optics 29 in the signal leg 31 serves to adjust the incoming beam to the size required to generate the desired optical channel shape. As the RF input signal is applied to the signal path Bragg cell, the resulting acoustic wave launched through that cell modulates and filters the signal path beam 31 to produce P deflected output beams. The deflection of each beam is proportional to the corresponding frequency component.
At the same time, a reference signal R(t), containing Q equally spaced known frequency components in the bandwidth of interest (e.g., 500 MHz), is applied to a transducer 41 of a `reference` Bragg cell 40, disposed in a `reference` leg or path portion 33 of the output beam from the laser 30. (To reduce the complexity of the illustration, only the center frequency beam is shown. In actuality there are Q diffracted beams exiting the reference Bragg cell 40, with the diffraction angle(s) being determined by the corresponding reference signal frequencies.)
As the reference signal R(t) modulates and filters the reference beam 33, it produces Q deflected output beams, each of which is deflected by an amount proportional to its corresponding frequency. A reference beam and a signal beam interfere when a deflected beam in the signal path lies in the same optical path as a deflected beam in the reference path.
Via a downstream beam splitter-combiner 23, the deflected output beams of the signal Bragg cell 20 sum or interfere with any corresponding deflected output beams of the reference Bragg cell 40. These beams are then focused via (Fourier) beam-projection optics 50, which typically comprise a set of lenses, so that, as diagrammatically illustrated in FIG. 2, a summed reference beam and a signal beam pair will be confined within the light receiving or sensitivity area of the respective photodetector 62 of a photodetector array 60. If there is no signal beam present to interfere with a given reference beam, the projection optics will confine only the corresponding reference beam within the light receiving or sensitivity area, i.e., unit 61, of the respective photodetector of photodetector array 60.
The combined beams are deflected to a location on the photodetector array 60 in accordance with the frequency content of the acousto-optically processed light beam and the Fourier optics 50, so that the respective detectors of the photodetector array 60 will be associated with successively adjacent bins or channels, each corresponding to a prescribed spectral portion of the overall system bandwidth. Namely, for a given RF system bandwidth BW and an array of N photodetectors, the imaging optics 50 and associated reference Bragg cell 40 with reference signal R(t) containing Q frequency components will be configured to place one reference beam spot on each of the N successively adjacent channels or bins, each of which has an individual bandwidth of BW/N Hz at a spatial resolution of Q spots per N photodetectors (or one spot per photodetector). As a non-limiting example, for an RF input signal bandwidth of 500 MHz (e.g., from 750 MHz to 1.25 GHz), an array of twenty-five photodetectors may be employed to subdivide the RF signal into twenty-five successively adjacent frequency bins, each having a bandwidth of 20 MHz, wherein the (IF) contents of a respective channel or bin are a coherent representation of the RF input signal.
Since the photodetector array and its associated post-detection signal processing electronics are relatively high cost components, that are often of modular configuration so that they may be housed within a constrained packaging platform, increasing the channelizer's signal processing bandwidth by some factor M, for example by a factor of two or more, can not only entail a considerable expense of adding more photodetector arrays and/or associated array output processing circuitry, but may not be practical or even possible from a hardware standpoint.